Over 400 papers have been published utilizing radioactive microspheres for determining blood flow distributions, yet microspheres have not been validated for use in small tissue regions. The goal of our investigation is to determine the accuracy limitations of varying size microspheres for determining red blood cell distribution in microvascular beds. We propose to characterize by vital microscopy and videodensitometric velocity profile measurements, the differences in distribution of varying size carbonized microspheres and red blood cell flow in different microvascular beds. Based on these results the accuracy limitations of carbonized microspheres for small tissue regions may be validated or defined. The study of the distribution of microspheres in relation to blood flow will also produce fundamental information regarding the rheology of both red blood cells and rigid particles in vivo.